Arc furnace and, more specifically, to means for automatically controlling



INVENTOR.

Filed July 6 1939 K. FREUDENHAMMER ARC FURNACE AND, MORE SPECIFIGALLY TO MEANS FOR AUTOMATICALLY CONTROLLING July 1, 1941.

Patentecl `Iuly 1, 1941 f ARC FURNACE AND, MORE SPEOIFICALLY,

TO MEANS FOR AU TROLLING TOMATICALLY CON- Klaus Freudenhammer, Berlin-Charlottenburg,

Germany, assignor to PatentverwertungsGesellschaft mit beschrnkter Haftung I Iermce, Berlin, Germany, a corporation ofGermany Application July 6, 1939, Serial No. 283,039 In Germany July 13, 1938 11 Claims.

ing of the electrode when in the operation of I the arc furnace the operating factors which determine and control the automatic adjustment of the electrodes have reached their normal or intended value.

A further object of the invention is to provide a mechanism for obtaining a definite adjustment of the control motor and for preventing hunting thereof when the impulse for controlling the operation of said motor disappears and the same tends to continue the adjusting motion by reason of its inertia.

Since electrodes which are automatically regulated must be suspended with as little friction as possible, a burning away of the electrodes and a resulting reduction in weight thereof easily causes a state of unbalance between the electrodes and their counterweights and incorrect adjustment of the electrodes.

It is therefore another object of the present invention to provide means for compensating for such loss-es in weight of the electrodes and for obtaining proper adjustment thereof at all times.

It has also been found that a too sensitive regulation of the control mechanism caused by small deviations of the operatingfactors from their normal value easily results in constant actuation of the control mechanism and undue wear thereof. y l

If for avoiding such undue shifting of the electrodes a relay is provided for short-circuiting the armature of the control motor when the operating factors have reached their normal value, such relay must withstand strong currents and operate frequently. However, since the operation of such relay must be interrupted frequently for checking and adjustments thereof, the

operation of the entire control mechanism will be likewise interrupted.

It is therefore another object of the present quires the control of low currents and Aavoids the necessity for a mechanical relay.

A feature of the invention for attaining the above objectives resides in coupling a direct current generator with the control motor so as to act as a brake thereon. For Ithis purpose, the generator is provided with a closed armature circuit and its iieldis excited only when the operating factors controlling the same have attained their normal value.

Other objects and features of the invention are the provision of electrically controlled vapor discharge devices for switching on and cutting off the exciting current of the brake generator, similar discharge devices for supplying the Icontrol motor with current, and means depending on .the operating factors for controlling said disvices also for switching on or cutting oif the exciting current of the' brake generator, .the grid circuits of the two groups of discharge devices may be so connected with each other that the same control voltage which ignites one group also effects a blocking or interruption of the other group.

Further objects, features and advantages will appear from the following detail description and the accompanying drawing, in which Fig. 1 is a diagrammatic showing of an embodiment of the present invention, and

Fig. 2 indicates a phase shifting according to the present invention .for controlling the operationof the brake generator.

With reference to Fig. 1 of the drawing, the control motor i is shown as a series-wound direct current motor having an amature 2 and two exciting windings 3 and tone of which is designed for righthand 'rotation and the other fcrlefthand rotation. The armature 2 of the control motor I is directly coupled with a generator l which functions as a brake. The armature of the brake generator motor I is short-circuited through a resistance 1 which is preferably adjustable so as to permit an adjustment of the braking action upon the control motor I. For increasing such braking action, the brake generator 5 is preferably designed to have a considerable output when rotating slowly; that is, it is a low speed generator. For supplying the control motor I and the exciting winding 6 of the brake generator with current, a discharge vessel 8 is provided which contains six anodes 9, 9 I0, I', II, II', their respective control grids 9a, 9b, Illa, IIb, IIa, IIb and a common cathode I3. The pair of anodes 9 and 9' is intended to control the righthand rotation and the anodes ID and vIll' the lefthand rotation of the control motor I, while the anodes II and II supply the exciting winding 6 of the brake generator with current. The-driving energy for the control motor I and the exciting current for the brake generator are supplied from a source of alternating current through a transformer I2 having three secondary windings I2a, I2b and I2C.

An arc furnace of the three phase type is shown diagrammatically at 50. The electrode 36 is adjustable in a vertical direction by means of a rope 39, the opposite ends of which are attached to the electrode support and to the counterweight 40, respectively. The rope passes over a pulley-38, which is mounted on an extension of the armature shaft of the control motor I. The apparatus shown provides only for adjustment of the electrode 36, and similar apparatus may be provided for each of the other electrodes.

In the embodiment shown in Fig. 1, the operation of the motor I and the brake generator 5 is controlled by the current of the arc I8 between electrode 36 and the melt 31, and in accordance with fluctuations thereof relative to a constant current. For this purpose, a transformer I9 is provided the primary winding of which is placed in the electrode circuit while its secondary winding is connected through another transformer 5| and a rectifier with a resistance I4 to supply the same with direct current. For obtaining a'4 normal value with which the current of the arc I8 may be compared, a variable resistance 2I is connected, on the one hand, to a source of constant alternating potential and, on the other hand, through a transformer 52 and a rectifier.

22 with the resistance I5. The difference in potential occurring on resistances I4 and I5, which are connected in series, controls the automatic adjustment of the electrode 36. This potential difference is zero as long as the current of the arc has its normal value, while, when the current of the arc deviates from its normal value, this potential becomes either positive or negative depending upon whether the deviation lies above or below the normal value.

This potential difference is then impressed upon the control grids 9a, 9b or I0a, I0b, respectively, relative to the cathode I3, by way of the choke coils 43 or 44 and the grid resistors 33 or 34, the cathode I3 being connected to the junction of resistances I6 and I1 by means of a resistor 24, If, for example, the potential difference is such that the right hand terminal of the resistance I4 is positive relative to the left hand terminal of the resistance I5, a current flow is established through the resistances I6 and I1 in such a direction that the left hand terminal of resistance I6 becomes positive with respect to the right hand terminal of resistance I1. The grids 9a, 9b are thereby made positive relative to the cathode I3 so that a current flows to the anodes 9, 9 causing righthand rotation of the motor I. During this time the flow of'current to the anodes II), I0 is interrupted since the potential of the control grids Ia, IIIb is shifted in a negative direction relative to the cathode I3. The opposite effect occurs if the left hand terminal of resistance I5 is positive relative to the right hand terminal of the resistance I4.

The same potential difference also serves for controlling the discharge group II, II', IIa, IIb which supplies the exciting winding 6 of the brake motor 5. For this purpose a phase shifting bridge arrangement 23 is provided containing two preexcited reactance coils 26 and 21 which are influenced by the potential difference in such a manner that, regardless of whether the potential diflerence is negative or positive, the phase position of the grid potential for the discharge group II, II is always shifted in the same direction, that is the direction in which the flow of current to the anodes II, II is blocked or interrupted.

Describing the phase shifting bridge and associated circuits more in detail, the arms of the bridge are made up of the two impedances 28 and 29 and two iron core choke coils 26 and 21. The cores of the coils 26 and 21 are provided with magnetizing windings 26 and 21', respectively, which are connected in series, and are supplied with current in parallel with the resistance I6-I1 when a difference of potential exists across the resistance I4-I5. Whenever current iiows in the windings 26 and 21', its effect is to reduce the inductance of the coils or windings 26 and 21, regardless of the direction of current ow. The bridge circuit is supplied with alternating current through a variable condenser 25. The bridge proper includes the primary winding of a transformer 30, the secondary winding of which is connected to the grids IIa and IIb through the grid resistors 35. The midpoint of the secondary winding of transformer 30 is connected to the cathode I3 through a resistor 3|, shunted by a condenser 32.

The phase position of the grid potential of discharge group I I when the operating factors are in normal condition may be preadjusted by the variable condenser 25 preferably in a manner as shown in Fig. 2 in which Ua indicates the alternating potential on the anode II or II'. The grid of the respective anode may have the constant negative or blocking bias Uv. The condenser 25 should be so adjusted that the grid potential supplied when in normal position by the bridge circuit 23, has a phase position indicated by the curve Ug. The ascending branch of the curve Us intersects with the zero line at exactly the same point where the ascending branch of the positive half wave of Ua passes through the zero line so that when in this condition the respective anode has attained its full efliciency. Obviously, the same is true for the other anode of thesame group.

A potential difference occurring on resistances I4 and I5 which preexcites the reactances 26 and 21, then effects a shifting of the curve of the grid potential Ug which, however, does not at first produce a change in the efficiency of the anode group II, I I'. This anode group ignites at the earliest possible time until the curve of the grid potential has reached the position indicated at Us. The brake generator therefore remains excited and prevents rotation of the control motor until the grid potential has attained this position. If, however, the potential difference increases to a still higher value, i. e., if the deviation of the operating factors of the arc from the normal value increases still further, the discharge groups II, II are blocked abruptly since no igniting grid potential remains for example in the position Us during the time of the positive half wave Ua. The braking action of the generator is therefore suddenly discontinued andd the control motor I may be actuated to produce the necessary adjustment of the electrodes.

In order to be sure that current ows through the control motor I only at such a time when no current flows through the exciting winding 6 of the brake generator 5 and when the braking action of the latter is released, a resistance 24 is provided in the exciting circuit of the winding 6. The drop in voltage hereby produced on resistance 24 shifts the potential of the control grids 9a, 9b or Illa, Ib, respectively, in a negative direction relative to the cathode I3. This shifting must be so large that the discharge groups 9, 9 or I0, Ill', respectively, cannot ignite as long as current flows through the exciting winding 6. The control potential on resistances I6 or I'I therefore cannot ignite one or the other discharge group 9, 9 or I0, I0 and actuate the control motor I through the exciting winding 6.

From the foregoing it will be seen that the exciting winding 6 of the brake generator 5 is normally energized. The circuit of winding 6 extends from the cathode I3 of discharge device 8 by way of anode II or Il', depending on which anode is positive, either the right hand half or left as long as a current flows hand half of secondary winding I2C, winding'B, v

and resistance 24 back to the cathode I3. Alternating potentials are applied to the grids Ila and IIb from the bridge circuit 23, which are normally near enough in phase with the anode potentials for ignition purposes, and the right hand section of the discharge device 8 therefore passes current and winding 6 is energized. The

. flow of current in the traced circuit of winding 6 produces a negative bias on the grids 9a, 9b, Ina, and |013, due to the drop across resistance 24, which prevents the control motor I from being started, even though there is a difference of potential at the opposite ends of resistance I6 -I1. A small difference in the current flow through resistances I4 and I5 therefore is ineffective to start the control motor.

Inequality in the current flow through resistances I4 and I5 is caused by an improper adjustment of arc I8. When any inequality exists a difference of potential is established which causes current to ow through resistance I6-II and also through the magnetizing windings 26' and 2l of the phase shifting bridge circuit 23, which causes the potentials applied to the grids I Ia and I Ib to be shifted in phase relative to the potentials applied to the associated anodes. The amount of the phase shift is in proportion to the amount by which the adjustment of the arc departs from normal, and accordingly increases as the' arc adjustment becomes less eilicient, due, for example, to burning away of the electrode 36. When a sulrlcient phase shift has occurred, the right hand section of the discharge device 8 can no longer pass current and the circuit of winding 6 of the brake generator 5 goes open. Since no current now fiows in resistance 24, the effect of this resistance in producing a negative bias on grids 9a, 9b, I0a, and IIlb is removed. Either grids 9a and 9b or I0a and Ib now instantly become positive, depending on the direction of -current flow in resistance I6I1, the correcomes conductive, and the control motor I is started.

It will be understood that the windings 3 and 4 are properly related to the control circuits so that the motor is run in the proper direction to correct the improper adjustment of the arc. If, for example, the arc has become improperly adjusted due to burning away of the electrode 36, the motor will be started up in such a direction as to lower the electrode. As the normal condition of the arc is thus restored, the grid potentials applied to grids Ila and IIb are again shifted in phase and at the proper time conductivity is again established in the right hand section of the discharge device 8. Current ow in the exciting winding 6 of the brake generator 5 is thus reestablished and since the generator armature is rotating a powerful braking effect is produced. At the same time that the generator is made effective the grids 9a, 9b, I0a, and I 0b are biased negatively again, rendering the two left hand sections of discharge device 8 nonconductive, and power to the control motor is shut off. Thus the motor is stopped quickly without overrun.

Although only one embodiment of the present invention has been described specifically, it should be understood that various modifications may be made therein without departing from the spirit and the scope of the invention as defined by the following claims.

l claim: l

1. An apparatus for automatically regulating the electrodes of an arc furnace comprising an electric motor for adjusting the position of the electrodes, two windings for said motor for causing it to rotate in opposite directions,.respec tively, means including two grid controlled rectiiier elements for supplying direct current to said windings, respectively, means for simultaneously placing opposite potentials on the grids of said elements, and means governed by the relation between the arcresistance and a known resistance with which it is compared for determining which element has a positive grid at any instant. l

2. An apparatus for automatically regulating the electrodes of an arc furnace comprising an electric motor for adjusting the position of the electrodes, two windings for said motor for causing it to rotate in opposite directions, respectively, means including two grid controlled rectier elements for supplying direct current to said windings, respectively, said elements having a common cathode, a resistance connected between the grids of said rectifier elements, a cathode connection to the midpoint of said resistance, and means for passing current through said resistance in one direction or the other depending on whether the current iiow through'the arc is greater or less than a predetermined value, whereby one or the other of said rectifier elements is rendered conductive.

3. An apparatus. for automatically regulating the electrodes of an arc furnace comprising an electric motor for adjusting the position of the electrodes, two windings for said motor for causing it to rotate in opposite directions, respectively, means including two grid controlled rectier elements for supplying direct current to said windings, respectively, means for simultaneously placing opposite potentials on the grids of said elements, means governed by the relation between the arc resistance and a known resistance with which it is compared, for determining which element has a positive grid at any instant and means for changing the value of the known resistance at will to vary the mean position of the electrodes.

4. Control apparatus for a reversible direct current motor used to adjust the position of the electrodes in an arc furnace, said apparatus comprising two space discharge devices for controlling the flow of current to the two field windings of said motor, respectively, means for comparing the current iiow through the arc with the current flow in a checking circuit of known characteristics, and means effective in case the difference between the arc current and the checking circuit current exceeds a predetermined amount for rending one or the other of said de vices conductive depending on which current is the greater.

5. Control apparatus for a reversible direct current motor used to adjust the position of the electrodes in an arc furnace, said apparatus comprising two space discharge devices for controlling the iiow of current to the two eld windings of said motor, respectively, a source of alternating current for the furnace, means for deriving a rectied current proportionate in value to the arc current, means for deriving another rectied current bearing a predetermined relation to the normal value of the arc current, means for comparing said rectied currents, and means for rendering one or the other of said devices conductive depending on the result of said comparison.

6. Apparatus for controlling a breaking device for the electrode adjusting motor of an arc furnace, said apparatus comprising a three element electronic valve for controlling current flow to said braking device, means for supplying grid potentials to said valve in phase with the anode potentials, whereby said valve passes current and said device is operative, and means effective in case of a maladjustment of the arc electrodes for shifting the phase relation of said grid and anode potentials to render said valve non-conductive and said device inoperative '7. Apparatus for controlling a breaking device for the electrode adjusting motor of an arc furnace, said apparatus comprising a three element electronic valve for controlling current flow to said braking device, means for supplying grid potentials to said valve in phase with the anode potentials, whereby said valve passes current and said device is operative, means responsive to an excessive increase or decrease in the arc current for starting said motor, and means also responsive to such increase or decrease for altering the phase relation between said grid and anode potentials to render said valve non-conductive and said braking device inoperative.

8. Apparatus for controlling a braking device for the electrode adjusting motor of an arc furnace, said apparatus comprising a three element electronic valve for controlling current flow to said braking device, means for supplying grid potentials to said valve in phase with the anode potentials, whereby said valve passes current and said device is operative, a control circuit for starting said motor, means for causing current flow in said circuit when the arc electrodes require adjustment, and means responsive to such current flow for shifting the phase relation between said grid and anode potentials and thereby render said braking device inoperative by stopping current iiow through said valve.

9. Apparatus for adjusting the electrode of an arc furnace, comprising a motor for moving the electrode, a braking device for quickly stopping the motor, means including a grid controlledv electronic valve for normally supplying current to said braking device, means for controlling the potential on the grid of said valve to interrupt the current supply responsive to a maladjustment of the electrode, and means responsive to such interruption for starting the said motor.

10. Apparatus for adjusting the electrodes of an arc furnace, comprising a motor for moving the electrodes, a. braking device for stopping the motor, two three element valves for controlling the supply of current to said motor and braking device, respectively, and a resistance common to the grid circuit of one valve and the anode circuit of the other valve, whereby the flow of current in said anode circuit places a biasing potential on the grid of the first valve.

11. Apparatus for automatically adjusting the electrodes of an arc furnace, comprising a motor for moving the electrodes, a generator having its shaft directly coupled to the shaft of said motor, a closed circuit for the armature of said generator, means for automatically starting and stopping the supply of current to said motor in accordance with the need for adjustment of the electrodes, and means for supplying current to the field winding of said generator whenever the supply of motor current is stopped, whereby the generator places a load on the motor which brings it to rest, and means for adjusting the resistance of the generator armature circuit.

KLAUS FREUDENHAMMER. 

